Biochemical studies have determined bone matrix lipids to consist of about 0.1% of bone by weight. Their ultrastructural location and relationship to collagen and mineral, their function and their origin are not known. Recent preliminary work by the investigator clearly show, by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray microanalysis (EDX), the presence of large quantities of intracellular (osteocytes) and extracellular lipids in the haversian canal and within the bone matrix of young, developing human osteons (see Progress Report). Tracing of lipids with radioactive choline in a young rat may prove or disprove the suspicion that bone matrix lipis migrate from the endosteal narrow cavity through the haversian canals (outside the capillaries) and finally become trapped by collagen secreted by osteoblasts as the osteon is formed. Whether they play a role in mineralization of the osteoid and thus bind Ca and/or P will be tested by autoradiography and EDX analysis of TEM sections containing Ca45 and/or P32. Digestion with lipase, phospholipase and chloroform already successfully employed in our preliminary SEM and TEM studies will also be used.